The present invention relates to vacuum pumps and more particularly to screw pumps.
Screw pumps usually comprise two spaced parallel shafts each carrying eternally threaded rotors, said shafts being mounted in a pump body such that the threads of the rotors intermesh. Close tolerances between the rotor threads at the points of intermeshing and with the internal surface of the pump body, which acts as a stator, causes volumes of gas being pumped between an inlet and an outlet to be trapped between the threads of the rotors and the internal surface and thereby urged through the pump as the rotors rotate.
Such screw pumps are potentially attractive since they can be manufactured with few working components and they have an ability to pump from a high vacuum environment at the inlet down to atmospheric pressure at the outlet.
The shafts of conventional screw pumps can be either mounted in cantilever fashion within the pump body or supported at each end with bearings using a common head plate or plates to support the bearing or bearings of both shafts. The head plate or plates are then fixed to the pump body.
Screw vacuum pumps are commonly used in the semiconductor industry and, as such, need to be capable of maintaining a clean environment associated with semiconductor device processing, especially in that area of the pumpxe2x80x94the pump inletxe2x80x94closest to the semiconductor processing chamber to which the pump is attached.
Screw vacuum pumps are known in which the rotors are positioned, and adapted for rotation, in the pump body by means of shaft bearings present wholly or partly inside hollow cavities in the rotors which are sealed at the ends closest to the pump inlet. A disadvantage of such pumps, however, is that the high rotational speeds of operation generate considerable heat, especially if the rotors/threads are designed to compress the volumes of gases as they pass through the pump. In other instances, particularly in the semiconductor industry, the pump is operated at high temperatures to avoid the possibility of certain substances being pumped, for example ammonium chloride, condensing on the internal surfaces of the plump.
Because of these high operating temperatures, the internal bearings inside the hollow rotors have to be cooled to prevent heat from the screw rotors from damaging the bearings. This can be achieved, for example, by water cooling the bearing carriers. However, the presence of such cooled bearing carrier surfaces within the pump allows for the possibility of condensation of condensable substances which is clearly undesirable as it may quickly hinder the efficient operation of the pump as a whole.
The present invention is concerned with the provision of an improved vacuum pump designed to overcome such disadvantages.
In accordance with the invention, there is provided a screw pump comprising a first shaft spaced from and parallel to a second shaft mounted in a pump body, a first rotor mounted on the first shaft and a second rotor mounted on the second shaft, each rotor having formed on an outer surface at least one helical vane or thread, the helical vanes or threads intermeshing together so that rotary movement of the shafts will cause a fluid to be pumped from an inlet towards an outlet of the pump, a first bearing arrangement associated with the first shaft and a second bearing arrangement is associated with the second shaft, first and second bearing arrangement being positioned in cavities within the first and second rotors respectively which are sealed at their ends closest to the pump inlet wherein a thermal shield is provided between the bearing arrangements and the internal cavity surfaces.
A thermal (or heat) shield is advantageously placed around the bearing carrier or carriers for each shaft. Preferably the shields are spaced apart from the bearings/bearing carriers to define a gap therebetween.
In preferred embodiments the thermal shield(s) comprises a tubular body surrounding the bearing(s) or bearing carrier(s).
In further preferred embodiments, the thermal shield includes seal means between it and the screw rotor in order to minimise the amount of pumped gases (or other contaminants) which might penetrate the cavity between the screw rotor and the thermal shield. This can be important because the end of the shield furthest into the screw rotor cavity is generally less hot than the end nearer the pump exhaust and the further end of the cavity is therefore more susceptible to condensation (or other deposition) by condensable substances.
A labyrinth seal is preferred for the seal between the thermal shield and the screw rotor, for example positioned on the thermal shield end adapted for close tolerance (non-contact) positioning relative to the (rotating internal rotor cavity surfaces.
To minimize the possibility of an ingress of powders or other particles past the seal, centrifugal means can be employed, for example, by having a plurality of blades at the end of the screw rotors angled to deflect any powder/particles outwardly and away from the rotor cavity, or by providing an angled separation between the rotor and the shield to spin the gas (and entrained powder/particles) away by a viscous drag mechanism instead of using blades.